Bulging processes have been employed to produce hollow formed bodies which are long and whose cross-sectional shapes and dimensions perpendicular to the longitudinal direction thereof differ from position to position (see, for example, Patent Document 1). As described in Patent Documents 2, 3, bulging apparatus for performing bulging processes have a single compressing mechanism that is combined with interchangeable dies. The bulging apparatus have a plurality of dies each interchangeable with another die suitable for a shape to be formed.
A bulging process using a straight tube as a blank workpiece will specifically be described below. First, the straight tube is gripped and placed in a die. Then, a fluid under pressure (generally water under high pressure) is supplied into the straight tube.
Therefore, the straight tube has its inner circumferential wall pressed by the fluid under pressure, and is expanded diametrically outwardly. As the straight tube is placed in the die, the expanded portion of the straight tube is finally stopped by the die. Therefore, the straight tube is formed into a shape corresponding to the cavity of the die. This process is also referred to as a tube expanding process.
Then, the die is removed from the compressing mechanism, and another die is mounted in the compressing mechanism for performing a next forming process. At this time, the die used in the tube expanding process is retracted from the compressing mechanism, and the die to be used in the forming process is moved to the compressing mechanism.
The expanded straight tube is transferred to the die, and then compressed to a predetermined shape by the compressing mechanism. A final formed product is now obtained.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-96118
Patent Document 2: Japanese Laid-Open Patent Publication No. 10-156429
Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-150048